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Properties of Linear Poly(Lactic Acid)/PolyethyleneGlycol Blends

K. Sungsanit, N. Kao, S.N. BhattacharyaSchool of Civil, Environmental and Chemical Engineering, Rheology and Materials Processing Centre,

RMIT University, Melbourne, Victoria 3001, Australia

Poly(lactic acid) (PLA) has great potentials to be proc-essed into films for packaging applications. However,film production is difficult to carry out due to the brit-tleness and low melt strength of PLA. In this investiga-tion, linear PLA (L-PLA) was plasticized with poly(ethyl-ene glycol) (PEG) having MW of 1000 g mol21 in variousPEG concentrations (0, 5, 10, 15, and 20 wt%). In rela-tion to plasticizer content, the impact resistance and

crystallinity of L-PLA was increased, whereas adecrease in glass transition temperature and lowerstiffness was observed. Nevertheless, the phase sepa-ration has been found in samples which contained PEGgreater than 10 wt%. The dynamic and shear rheologi-cal studies showed that the plasticized PLA possessedlower viscosity and more pronounced elastic proper-ties than that of pure PLA. Both storage and loss mod-uli decreased with PEG loading at all frequencies whilestorage modulus exhibited weak frequency depend-ence with increasing PEG content. POLYM. ENG. SCI.,52:108116, 2012. 2011 Society of Plastics Engineers

INTRODUCTION

Poly(lactic acid) (PLA) is produced from renewable

resources that has become a useful material, especially in

packaging applications due to its good clarity, high

strength, and moderate barrier properties. PLA resins have

mostly been used for biomedical applications such as drug

delivery systems and packaging applications such as films

and food containers [1]. Some of the environmental bene-

fits of PLA and opportunities for the future are presented

by Henton et al. [2]. These include PLA requiring less

energy to produce as well as reduced green house gas

production. Generally, commercial grades PLA are

copolymers of poly(L-lactic acid)(PLLA) and poly(D,L-lac-

tic acid) (PDLLA), which are produced from L-lactides

and D,L-lactides, respectively. PLLA is a semicrystalline

polymer whereas PDLLA is an amorphous polymer.

PLAs use in film packaging applications is highly de-

sirable due to its environmentally friendly nature. How-

ever, this requires film extrusion of PLA to be performed,

which is difficult to process due to the brittleness and low

melt strength of PLA. The rheological and mechanical

properties of PLA may be enhanced by blending it with a

plasticizer or with a second polymer. For instance, it could

be blended with other polymers such as linear low density

polyethylene [3], poly(vinyl acetate) [4] and polyethylene

glycol (PEG) [5, 6]. Possible plasticizers for PLA include

oligomeric lactic acid, lactide and low molecular weight

esters such as citrates [7]. These blends have been shown to

improve the flexibility of PLA. However, some of the

above-mentioned polymers are not biodegradable; hence,

they could not be considered as a possible blend with PLA

in this investigation. In consumer goods packaging applica-

tions, only nontoxic substances approved for food contact

can be considered as plasticizing agents. The selection of a

plasticizer to be used in a specific PLA composition

requires the consideration of many criteria: compatibility,

low volatility, resistance to migration, extraction during

service life, lack of toxicity, etc. [8].

Low molecular weight PEG is the most suitable mate-

rial to be classed as an impact modifier for PLA due to

its miscibility, biodegradability, and food contactable

applications [6, 911]. Jacobsen et al. [9] investigated the

plasticized PLA, with 2.510 wt% of PEG (MW 1.5 310

3) by melt blending. They found that the addition of

PEG to PLA led to a decrease of both tensile strength and

elasticity modulus but an increase of percentage elonga-

tion at break. Adding 10 wt% PEG resulted in an

enhanced impact resistance of more than five times that

of a pure PLA. Kulinski et al. [11] investigated the blend-

ing of semicrystalline and amorphous PLA with 5 and 10

wt% of PEG. They reported that at 10 wt%, an amor-

phous plasticized PLA could be deformed to about 550%

while a semicrystalline PLA exhibited nonuniform plasti-

cization of the amorphous phase and showed less ability

to the plastic deformation. Sheth et al. [5] have found that

PLA/PEG blends varied from completely miscible to par-

tially miscible, depending on the PEG concentration.

Below 50 wt% PEG content, the blends achieved the

higher elongations and lower modulus values. Above 50

wt% PEG content, the blend morphology was driven by

K. Sungsanit is currently at Rajamangala University of Technology

Thanyaburi, Pathumthani, Thailand.

Correspondence to: K. Sungsanit; e-mail: [email protected]

DOI 10.1002/pen.22052

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2011 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE-2012

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the increasing crystallinity of PEG, resulting in an

increase in modulus and a corresponding decrease in elon-

gation at break. Recent studies [6, 10] of PLA plasticized

with PEG in various contents have shown a limit of mis-

cibility of polymer blends. The PLA blended to PEGsbecame very brittle as a function of plasticizer content

and molecular weight. The plasticizing efficiency

increased with decreasing molecular weight of PEG. In

contrast, at the same molecular weight of PEG, material

became brittle at higher content because of a lack ofcohesion between the separate phases.

This article investigates the effect of plasticization on

the properties of L-PLA/PEG blends at various PEG con-

tents. Differential scanning calorimetry (DSC) was carried

out in a wide range of temperatures in order to evaluate

crystallization and melting behavior of the blends. Fur-

thermore, impact and tensile properties were investigated

to evaluate the outcome of PLA plasticization with PEGfor film applications. Surprisingly, there is very little liter-

ature examining the plasticizing effects of PEG on the

rheological properties in terms of steady shear and

dynamic measurements of PLA/PEG blends which play

important roles in polymer processing. Thus, the melt rhe-

ology of pure L-PLA and plasticized L-PLA was con-

ducted in this study and the results are presented below.

EXPERIMENTAL

Materials

The poly(lactic acid) (4032D-grade) from NatureWorksproduced by Cargill Dow LLC, used in this study, com-

prised of 2% D-LA content. Polyethylene glycol (molecular

weight 1000 g/mol), a food-contact approved grade from

Sigma-Aldrich, was chosen as a plasticizer for the L-PLA.

The materials used and their properties are listed in Table 1.

Blending Preparation

L-PLA pellets were dried in an oven under vacuum at

508C overnight (1215 h) prior to blending. Drying was

necessary to minimize the hydrolytic degradation of the

polymers during melt processing in the extruder. L-PLA

and PEG were melt-blended using a Brabender twin-

screw extruder in the ratio of 100/0, 95/5, 90/10, 85/15,

and 80/20 where the first and second number represent L-

PLA and PEG by weight percentage, respectively. For

better comparison, the pure L-PLA sample was also proc-

essed in the same twin screw extruder and processing

conditions to ensure identical thermal history with thoseof L-PLA/PEG blends. Dry-mixing of each polymer was

first carried out in a zip-lock bag before blending. The

twin screw extruder had a screw diameter of 17.8 mm

and an L/D ratio of 40. The extruder had three controlled

temperature zones which were set from 1808C (next to

the feeding segment), 1908C (compression zone) and

2008C (metering zone). The screw speed was maintained

at 30 rpm for all runs. Subsequently, plasticized L-PLA

pellets were dried again under vacuum at 508C overnight

prior to sample preparation by injection and compression

molding.

Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) was carried

out with a DSC TA Instrument 2920. The samples were

preliminarily heated to 473 K to discard any anterior ther-

mal history and held at that temperature for 5 min. It was

then cooled to 253 K at a rate of 10 K min21

and kept at

253 K for 5 min before a second heating scan from 253

to 473 K at 10 K min21

scan rate was carried out. During

the second heating scan the glass transition, cold crystalli-

zation and melting temperature of the material could be

determined; whilst the crystallization temperature was

determined from the cooling scan. The degree of crystal-linity of all samples were calculated by

Crystallinity % DHm DHcc=UPLA

93:6 100: (1)

Most commonly, an enthalpy of fusion of 93.6 J g21

is

used for a 100% crystalline PLLA or PDLA homopoly-

mers [12], where DHm is the measured heat of fusion and

DHcc is the heat of cold crystallization. This value is used

throughout the PLA literature. FPLA is the PLA content in

the component.

Mechanical Characterization

Tensile measurements were carried out to determine

the tensile strength, tensile modulus and % strain at break

using the Instron 4467 Universal testing machine at room

temperature. The testing was carried out at a rate of 5

mm min21 according to ASTM D638 on standard Type I

dog-bone shaped samples with sample thickness of 4.1

mm.

The impact resistance was determined for each mate-

rial with 10 samples, which were tested under the temper-

TABLE 1. Properties of PLA and plasticizer used in this study.

Materials

Mw(g/mol)

Tm(K)c

Tg(K)c

Chemical

formula

Linear poly

(lactic acid)

(L-PLA)

155,000a 442 334

Poly(ethylene

glycol) (PEG)

1,000b 313 203

aDetermined by Gel Permeation Chromatography (GPC) versus

polystyrene standards.b As stated by the manufacturer.c

Determined by Differential Scanning Calorimetry (DSC).

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE-2012 109

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ature like the tensile test and following the ASTM stand-

ard specification ASTM D256, the so-called izod method,

with a Davenport impact tester. The samples were

notched and a pendulum hammer of 1.36 J was used.

Both tensile and impact samples were injection molded

and were dried in the vacuum oven at 508C over night

prior to testing.

Rheological Characterization

The rheological properties of L-PLA with various PEG

contents were measured using an Advanced RheometricExpansion System (ARES) fitted with a 25 mm parallel

plate geometry. Tests were performed at 1808C under a

nitrogen atmosphere to avoid any degradation. Sample

disks for the rheometer were compression molded at

2008C into 25 mm diameter discs $3.2 mm thick andwere again dried in the vacuum oven overnight at 508C

prior to testing.

Dynamic strain sweep tests were carried out to confirm

the linearity of the viscoelastic region up to 100% strain

at 10 rad s21 frequency. Frequency sweeps were carried

out to determine the dynamic moduli and complex viscos-

ity over a frequency range of 0.1100 rad s21

at 10%

strain. All tests were started when the temperature had

stabilized after loading the sample into the rheometer.

Steady shear measurements were performed over a shear

rate range of 0.0110 s21

.

Fractured Surface CharacterizationThe fractured surfaces of all materials were observed

by Environmental Scanning Electron Microscopy

(ESEM), using a Philips XL-20 SEM at an accelerating

voltage of 30 kV. All of the samples were fractured speci-

mens after impact tests. The fractured surface was then

coated with a thin layer of gold prior to observation.

RESULTS AND DISCUSSION

Thermal Properties

The results obtained from differential scanning calo-

rimetry of the pure L-PLA (unprocessed and 0 wt% PEG)

and plasticized L-PLA are shown in Fig. 1 and important

numerical values are summarized in Table 2.

Table 2 summarizes the glass transition temperature

(Tg) of plasticized L-PLA as well as crystallization and

melting temperatures (Tc and Tm). Pure PLA showed a

sharp Tg and its magnitude decreased gradually with

increasing PEG concentration. A minor decrease in the

melting temperature (Tm) of 3 to 4 K was also observed,

indicating that the melting temperature of L-PLA was not

greatly affected by the addition of PEG. According to

DSC thermograms (Fig. 1a), the crystallization tempera-ture (Tc) seemed to be increased with increasing PEG

content. This was due to the presence of the plasticizer,

which facilitated the crystallization process of PLA. As

noted previously, the increased molecular mobility

increased the rate of crystallization, which allowed L-

PLA to crystallize to a higher degree during cooling [1]

and allowing the crystallization of L-PLA to occur at a

higher temperature (i.e., with less sub-cooling). This is in

accordance with the crystallization behavior of PLA as

reported by Miyata et al. [13]. However, there was an

irregular characteristic of Tc which showed sudden

decrease from 369 K to 363 K in the presence of a small

amount of plasticizer at 5 wt% PEG. It is presumed thatduring the non-isothermal crystallization temperature from

473 K to 253 K, L-PLA initially crystallized before the

formation of PEG crystals at temperature range between

373 K and 363 K. On the other hand, PEG crystallized

following the formation of L-PLA crystals between 303 K

and 293 K (determined by DSC). Consequently, during

the formation of L-PLA crystals some molecules of PEG

probably could be trapped in the intra-spherulitic region

of L-PLA and led to hindering the crystallization of L-

PLA. Nevertheless, addition more wt% of PEG enhances

FIG. 1. Effect of PEG concentration on crystallization and melting of

L-PLA/PEG blends: (a) cooling thermograms obtained with a cooling

rate of 10 K min21

; (b) subsequent heating thermograms obtained with a

heating rate of 10 K min21

.

110 POLYMER ENGINEERING AND SCIENCE-2012 DOI 10.1002/pen

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degree of the crystallization rate of L-PLA as seen in

Table 2.

In contrast, as seen in Fig. 1b for 0 wt% PEG, the Tgwas observed to be approximately 335 K. At 5 wt% PEG

the Tg decreased to approximately 319 K and down to

306 K when up to 15 wt% PEG was added. However, for

blend compositions having higher concentration of PEG,

there was no change in the Tg of L-PLA/PEG blends.

Moreover, the plasticized L-PLA showed crystallinity

between 36% and 44% for all the four different polyethyl-

ene glycol contents examined. Fig. 1b shows that the cold

crystallization temperature (Tcc) of L-PLA decreased in

the presence of the plasticizer. For the un-processed L-

PLA, the thermograms did not show the cold crystalliza-

tion peak since the material did not go through the ther-

mal process. While the processed L-PLA without PEG (0

wt % of PEG) showed the cold crystallization temperature

of 373 K and suddenly decreased to 263 K for the L-

PLA/PEG sample containing 10 wt% of PEG. For sam-

ples containing higher PEG content, cold crystallization

was no longer visible. In the present blends, comparedwith that of pure L-PLA, the depression of Tg and the sig-

nificant decrease in Tcc indicated that polyethylene glycol

was compatible and effective with L-PLA. It clearly

appeared that the decreasing of Tcc and Tg of L-PLA due

to enhanced chain mobility as the plasticizer level

increased. Pillin et al. [6] has also reported this behavior.

It was also interesting to see that the small melting and

crystallization peaks of PEG were seen for L-PLA/PEG

blends containing 15 and 20 wt% plasticizer concentration

(in the circle of Fig. 1a and b) instead of the observation

of the glass transition of blends. It is possible that there

was the phase-separation of pure PEG in this blend (com-

pared with melting temperature of PEG 1000 in Table 1).It is obvious that to facilitate the plastic deformation of

semicrystalline polymer, with glassy amorphous phase,

the plasticization of the latter and sufficient decrease of

Tg are required. However, crystallization while increasing

the average plasticizer content in the amorphous phase

may also induce a rejection of plasticizer from growing

spherulites and its accumulation at interspherulitic boun-

daries [14]. In the L-PLA/PEG blends investigated in this

work, segregation of a pure PEG phase always occurred

at plasticizer content higher than 10 wt%. From the DSC

analysis, it could be concluded that the PEG plasticizer

has ability to increase ductility of L-PLA by increasing

the mobility of L-PLA molecules.

Tensile Properties

The objective of plasticization is to improve the ductil-

ity while maintaining the blends strength and stiffness.

Results of tensile experiments are shown in Figs. 24.

When L-PLA sample was stretched without PEG, the ten-

sile strength, tensile modulus and % tensile strain at break

were 49 MPa, 4.3 GPa, and 1.3%, respectively. For plasti-

cized L-PLA at 5 wt% and 10 wt% PEG, the tensile

strength was 56 and 51, MPa, respectively. The higher

tensile strength values were due to the higher crystallinity

content than that of the L-PLA with 0 wt% of PEG

(whose tensile strength was only 49 MPa). Tensile

strength of the L-PLA/PEG blends gradually decreased to

29 and 22 MPa, when higher concentrations (15 and 20

wt %, respectively) of PEG were used (see Fig. 2). From

thermal characterization, it was clear that phase separationof L-PLA/PEG blends in this investigation could be

observed at PEG concentration of more than 10 wt%.

Moreover, from Fig. 3, a gradual decrease in the tensile

TABLE 2. Results from DSC for the L-PLA/PEG Blends.

PEG content (wt%)

Cooling Subsequent heating

Tc (K) DH (J g21) Tg (K) Tcc (K) DH (J g

21) Tm (K) DH (J g21) Xc (%)

Un-processed PLA 334 442 12 13

0 369 12 335 373 12 444 38 27

5 363 13 319 365 7 443 39 36

10 368 21 311 363 1 443 34 40

15 371 30 306 441 30 3920 378 30 306 440 33 44

All data are the average data.

FIG. 2. Effect of PEG concentration on tensile strength of L-PLA/PEG

blends.


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modulus from 4.3 to 1.9 GPa was observed as the concen-

tration of PEG increased from 0 to 20 wt% in the L-PLA/

PEG blends.

In the meantime, the tensile strain at break (see Fig. 4)

increased from 1.5% to 15% with increasing PEG content

of 0 wt% and 20 wt% PEG, respectively.

Impact Properties

The impact resistance properties of L-PLA/PEG blends

are shown in Fig. 5. Clearly, it could be seen that the

presence of PEG as a plasticizer in L-PLA marginally

increases the toughness of L-PLA. For L-PLA/PEG bland

(0 wt% PEG), the average value was 13.3 kJ/m2 but the

data increased dramatically to 15.4 and 14.9 kJ m22 for

L-PLA/PEG samples containing 5 and 10 wt% PEG,

respectively. As expected, the increase in impact strength

of L-PLA was observed when plasticized with PEG

because the plasticizer interacted with the polymer chainsdistributing itself uniformly within the polymer, hence

creating additional free volume. However, the decrease in

impact strength was also observed when PEG concentra-

tion reached 15 and 20 wt% due to separation of PLA

and PEG phases as seen from thermal characterization.

This indicated that the blends were ductile at less than 10

wt% plasticizer content and were brittle at greater than 15

wt% plasticizer content.

It should be noted that it is important to have informa-

tion about the impact resistance behavior at other defor-

mation rates as well. Because of some differences

between tensile and impact deformation rates, it has beenfound in this investigation that blends with high elonga-

tion at break were characterized by relatively weak

impact values. Therefore, it is important to note that for

plasticized blends with high content of plasticizer (PEG)

a decrease in impact strength has been reported. Jacobsen

and Fritz [9] attributed this to a disturbance created by

the plasticizer composition into PLA matrix.

Surprisingly, the abrupt change of all mechanical prop-

erties occurred regularly in L-PLA/PEG blends whose

PEG concentrations were greater than 10 wt%. For higher

PEG concentrations, the material became brittle due to an

occurrence of phase separation at the saturated points of

PEG content. This could result only from a phase separa-tion in the amorphous phase leading to the formation of a

plasticizer rich phase and depleting the PLA of plasti-

cizer. SEM studies of fractured surfaces of impact bar as

shown in Fig. 6 illustrated some emptied voids and round

white particles in L-PLA/PEG blends (15 and 20 wt% of

PEG) where PEG accumulated during phase separation.

The fractured surfaces represented the dispersion of PEG

phases in L-PLA matrix uniformly and the size of the

PEG domains was of sub-micrometer order. Additionally,

the rich PEG phase and smooth plane could be seen moreFIG. 4. Effect of PEG concentration on % strain at break of L-PLA/

PEG blends.

FIG. 5. Effect of PEG concentration on the impact strength of L-PLA/

PEG blends.

FIG. 3. Effect of PEG concentration on tensile modulus of L-PLA/PEG

blends.


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clearly at the fractured surface of 20 wt% PEG, indicating

the phase separation and brittle failure of L-PLA/PEG

blends. Furthermore, the phase separation in the amor-

phous phase was already reflected in additional low tem-

perature glass transitions of the blends and in melting

peaks of PEG crystals in L-PLA/PEG blends evidenced

by the DSC measurements as shown in Fig. 1b. Upon

phase separation the PEG plasticizer accumulated in dis-

tinct pools.

Fractured Surface Analysis by SEM

The SEM micrographs revealed rather brittle fracture

of L-PLA/PEG blend (0 wt% PEG) with little amount of

plastic deformation with significant lateral contraction of

the test bar and many striated ridges as shown in Fig. 6

(15003 magnification). The lateral contraction and ridgesare the morphological manifestation of the fact that shear-

yielding had occurred during the impact test, resulting in

a ductile break. Figure 6 SEM micrographs of the impact-

fractured surfaces show more evidences of ductile frac-

tures as more and longer fibrils can be observed from the

surfaces with the increase in PEG content. Ductile fibril

formations were observed on the impact surfaces as

shown in Fig. 6ac. In contrast, these fibrils were

observed barely in sample without PEG at $5 lm lengthbetween the striated ridges and there was no longer fibril

on fractured surfaces of PLA/PEG blends (15 and 20 wt%

PEG). It has been reported [15] that this kind of fibril for-

mation appeared to be related to the increase of the tem-

perature in the crack-tip region above the glass transition

temperature due to heat generation at high strain-rate. The

Fig. 6b and c. shows the surface of the blend at 510

wt% PEG. There were more fibrils and roughness on the

surface compared with sample without PEG. This proved

that the PEG was equally dispersed into L-PLA matrix,which made the sample slightly more ductile. When PEG

content was above 10 wt%, the small cavities and white

round shapes of PEG were observed in Fig. 6d and e. The

white round shapes showed broad distribution at 15 wt%

PEG content. The distribution of the white round shapes

increased (as shown in Fig. 6e) with increasing PEG con-

tent (20 wt%). In addition, the sample containing 20 wt%

PEG content also showed a smooth fractured surface indi-

cating brittle fractured areas with considerable voids of

submicron size are clearly observed, which was probably

caused by the accumulation of PEG during phase separa-

tion.

Rheological Properties

To fully understand the processing properties of plasti-

cized L-PLA blends, a detailed investigation of the rheo-

logical behavior of these L-PLA/PEG blends with varying

PEG concentration was necessary.

Figure 7 presents the dynamic viscosity properties on

frequency at various PEG concentrations at 1808C. The

unprocessed L-PLA data were obtained from dynamic fre-

quency sweep measurement carried out on the nonex-

truded pellets. It exhibited a clear Newtonian Plateau atlow oscillation frequency with a zero-shear rate viscosity

around 1000 Pa s and seemed to be shear thinning behav-

ior at high oscillation frequency. On the other hand, the

processed L-PLA also exhibited a non-Newtonian behav-

ior, but with a much lower zero-shear rate viscosity value

(around 840 Pa s). All L-PLA/PEG blends of varying

FIG. 6. SEM micrographs (15003) of the fractured surfaces of the L-

PLA/PEG blends with (a) 0, (b) 5, (c) 10, (d) 15, and (e) 20 wt% PEG.

FIG. 7. Dependence of dynamic viscosity on frequency at various PEG

concentrations at 1808C.


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PEG concentrations exhibited a more pronounced Newto-

nian response with an extended Newtonian plateau com-

pared with unprocessed PLA and also showed the

decreasing zero-shear viscosity values as the PEG concen-

tration increased. This was the effect of more disentangle-ment of PLA chain due to increased segmental mobility

of PLA chains.

Additionally, it could be noticed from Fig. 7 that by

blending in the twin screw extruder the polymer was

exposed to excessively high shear strains resulting in a

greater degradation of the PLA. Moreover, the degrada-

tion in PLA during processing in the presence of plasti-

cizers with ester groups could also be due to potential

transesterification reactions leading to a decrease of PLA

molecular weights [16] which resulted in a decrease in

PLA viscosity.

The corresponding storage and loss moduli for theseblends are shown in Figs. 8 and 9, respectively. As

expected, the moduli of L-PLA decreased with increasing

plasticizer loading at all frequencies. Unprocessed L-PLA

and plasticized L-PLA (0-20 wt% of PEG) exhibited the

rheological behavior of a typical polymer melt as charac-

terized by a storage modulus (G0, Fig. 8) smaller than the

loss modulus (G00, Fig. 9). Both the G0 and G00 decreased

with increasing PEG concentration. However, at low-fre-

quency G0 of all blends presented lower frequency de-

pendency. Only at high frequencies, all samples approxi-

mately showed a common storage modulus. Such a non-

terminal behavior, sometime occurs at a medium

frequency region, as has been observed on many polymerblends and is accepted to be attributed to the change of

the shape of the discrete phase in the polymer matrix dur-

ing the oscillatory shear deformation, namely shape relax-

ation [17, 18].

In this investigation, at medium to low frequency

region the storage modulus exhibited weak frequency de-

pendency with increasing PEG content, such that there

were gradual changes of behavior from liquid-like (G0(x)

! x2

) to solid-like with increasing PEG content. At

frequency less than 1 rad s21

, the frequency dependent

transition of the blend with PEG concentration less than

10 wt% could be observed. On the other hand, the

frequency dependent transition of L-PLA/PEG blends at

PEG concentration higher than 10 wt% showed a medium

frequency dependent region between 1 and 10 rad s21. It

could be concluded that at higher PEG concentration the

G0 curves exhibited a plateau distinctly at the low fre-

quencies as the blends seemed to be a solid like behavior.

However it illustrated the discrete phase as well in thematrix if the plasticizer saturation point was reached. As

seen in Fig. 8, the slope of log G0 vs. log x for the unpro-

cessed L-PLA was close to 2, similar to the thermo-rheo-

logically simple polymer in the terminal regime. In con-

trast, the slopes of the storage moduli, in the terminal

region of low frequencies (0.11 rad s21), for L-PLA/

PEG blends were much smaller than 1 (in fact close to

0.5), especially for the blends containing 15 wt% and 20

wt% of PEG. Zheng et al. and Du et al., [19, 20] reported

that the experimental values of the slope for G0 obtained

from other phase separated or degraded polymer blends

varied between 0.5 and 1. Therefore, the small values of

these exponents suggested that the high concentration of

PEG may have contributed to the phase separation in

these blends as verified in the thermal and mechanical

characterization.

In addition, the use of a plasticizer reduces the inter-

molecular force and increases the mobility of the poly-

meric chains, thereby improving the flexibility and the

extensibility of the lasticized polymer [21].

The dependences of the dynamic loss moduli of L-PLA/

PEG blends on frequency (see Fig. 9) indicated that the

blends with higher PEG content had lower G00 values than

that of unprocessed L-PLA over the frequency range cov-

ered. This was due to the fact that G00

represented the vis-cous behavior (i.e., the amount of energy dissipated), and

the addition of PEG to the L-PLA produced a material with

the lowest energy dissipation. From the view of miscibility

of blends, the interaction between blends decreased to a

certain extent at higher PEG content. Hence decreasing in

FIG. 8. Dependence of storage modulus (G0) on frequency at various

PEG concentration in L-PLA/PEG blends at 1808C.

FIG. 9. Dependence of loss modulus (G00) on frequency at various PEG

concentration in L- PLA/PEG blends at 1808C.


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amount of energy dissipated of blends melting with all load

resulted in the decreased loss modulus [22].

Figure 10 shows the steady shear viscosity of L-PLA/

PEG blends at various PEG content. As expected both

unplasticized and plasticized L-PLA melts behaved as

typical non-Newtonian fluids. At all shear rates, the shear

viscosities of L-PLA/PEG blends were lower than those

of pure L-PLA melt and decreased considerably with

increasing PEG content. The slight shear-thinning behav-ior could be observed in unplasticized and plasticized L-

PLA in whole the shear region. The calculated parameters

from the measured rheological properties are summarized

in Table 3. All parameters were obtained by fitting the

modified Cross model as follows [23]:

Z Z0

1 t0gm

; (2)

where Z0 represents the zero shear rate viscosity (Pa s), t0represents the characteristic relaxation time (s), and m

characterizes the slope of the line over the pseudoplastic

region in the logarithmic plot.The Modified Cross model fits the data well. All the

equations have correlation coefficient (r2

) close to 0.95.

As shown in Table 3, the incorporation of PEG led to the

decreasing value of m and decreasing in s0. This referred

that adding more PEG content into L-PLA could lead to

decreasing in relaxation time and slight shear thinning

tendency. This was attributed to the addition of PEG was

easier to cover with PLA chains and the chain of PLA

was disentangled under higher shear rate. Moreover, PLA

blending with PEG also improved the elastomeric behav-

ior of matrix, which would resist the flow and make the

value of m tend to decrease.

As shown in Table 3, g0 decreased significantly with

increasing PEG content and the saturated points of PEG

content seem to be reached resulting in phase separation

[24]. This effect was observed in the PEG concentration

at 15 wt% and 20 wt%. According to theory [25], this

reduction in viscosity can be interpreted as an enhanced

mobility of polymer chain in the system.

CONCLUSIONS

This article demonstrated that plasticizing L-PLA with

PEG could produce a more flexible material with different

mechanical and rheological properties. It was found that

PLA/PEG blends lowered the glass transition temperature

and modified the crystallization properties. It clearly

appeared that the blends obtain a miscibility of the com-

ponents at 5 wt% and 10 wt% PEG content. Mechanical

characteristics of these materials showed a decrease in

modulus and stress at break, but an increase in % elonga-tion at break and impact strength. Nevertheless, L-PLA

blended with PEG became very brittle at higher plasti-

cizer content due to phase separation of PEG phase as

evidence from SEM micrographs.

Rheological study concluded that both unplasticized

and plasticized L-PLA exhibited a slightly shear thinning

behavior at all frequency regions. Viscosities of blends

decreased as PEG content was increased due to increasing

in chain mobility in the system. Moreover, during proc-

essing in the presence of plasticizers with ester groups

could also be potential trans-esterification reactions lead-

ing to a decrease of PLA molecular weights. The moduli

of PLA/PEG blends decreased with increasing plasticizerloading at all frequencies.

At low frequency region, the storage modulus exhib-

ited weak frequency dependence with increasing PEG

content. On the other hand, loss modulus decreased

monotonically with increasing plasticizer loading at all

frequencies.

In this article, plasticizing PEG with linear PLA just

have ability to improve the impact strength. Therefore

with this study we indicate a point of attention for plasti-

cizing branched PLA. A study probing the effect of PEG

on the properties of branched PLA is already in progress.

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FIG. 10. Dependence of shear viscosity on shear rate at various PEG

concentration of L-PLA/PEG blends at 1808C.


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